There are a number of traditional tactile measuring instruments available that are used by persons with blindness to measure lengths and angles in order to interpret or produce geometric features and constructions. Available as rulers and protractors, such instruments have tactile markings permanently etched, embossed or engraved on their surface. Adjacent tactile markings are spaced adequately so that users are able to distinguish between them by touch. Traditional instruments are therefore limited in precision. Precision of an instrument is related to its least count, which is the smallest change in measured dimension that can be resolved with the use of the instrument. Precision is higher for instruments with relatively smaller least counts. Least count of traditional tactile rulers is usually equal to or greater than half of a centimeter or to one-fourth of an inch, depending on the unit of measurement. That of traditional tactile protractors is usually equal to or great than five degrees. A few that have smaller least counts are difficult to use by touch. Some such traditional instruments also have embossed Braille characters accompanying the tactile markings to indicate the value of measurement at corresponding markings Due to the size of Braille characters and the limited space available, they accompany only some select markings. For other markings, value of the measurement is identified by counting the number of preceding markings To a Braille user, counting may be slower than reading Braille and may require more mental effort and concentration. Thereby, the use of traditional instruments is slow and inconvenient.
A few tactile measuring instruments give precision higher than that of the traditional ones. Some such rulers have a threaded shaft with pitch length equal to the desired least count. Auditory or tactile cues are generated mechanically with each rotation of the shaft. These cues are counted to identify the measurement. Other such rulers, as described in U.S. Pat. No. 4,328,618 and U.S. Pat. No. 4,614,042, use gauge elements or tabs with one physical dimension equal to the least count. Unlike traditional tactile measuring instruments, such instruments may have a smaller least count and therefore higher precision. However, they are usually larger and heavier. Further, Braille characters, if any, indicate only coarse values of measurement. Fine values of measurement are still identified by counting tactile markings or auditory cues. Thereby, similar to traditional instruments, their use is also slow and inconvenient.
Some measuring instruments have electronic systems that read out measurements either as discrete beeps or as synthetic speech. Compared to tactile measuring instruments, these auditory instruments have a higher accuracy. However, they run on a battery or an external power source, and are not suitable for all environments and situations of use. For instance, they are difficult to use in noisy environments and also when the user has other active sources of auditory information. Further, they are usually more expensive and delicate.
The technique and designs of this invention, on the other hand, readily and simply overcome these limitations both in the area of tactile measuring instruments and in the more general area of condensed representation of tactile forms. They preferably involve the use of a guide, a sliding jaw and a display mechanism for presenting values of measurements in Braille or other appropriate tactile forms. They are such that the accuracy of measurement is not limited either by the user's ability to distinguish between adjacent tactile markings or by the size of Braille characters. The effect of this is that the instruments may have a smaller least count and therefore higher precision. Braille characters, or other appropriate tactile forms, indicate the complete value of measurement rather than only the coarse value. This overcomes the need to count individual markings, which otherwise makes instruments slow and inconvenient to use. Auditory cues, generated with every change of measurement, supplement the tactile output. This informs users of desired or undesired changes in measurement and thereby prevents errors. With only a few small parts, the instrument is not large and heavy. Unlike measuring instruments with electronic systems, it does not require a battery or an external power source. Also, it is not as expensive and is less delicate. Further, it can be used even in noisy environments and also when users have other active sources of auditory information.